Electrospray involves breaking the meniscus of a charged liquid formed at the end of a capillary tube into fine droplets using an electric field. The electric field induced between the electrode and the conducting liquid initially causes a Taylor cone to form at the tip of the tube where the field becomes concentrated. Fluctuations cause the cone tip to break up into fine droplets, and Coulomb interaction between neighboring liquid ions causes them to separate from one another while being pulled towards the electrode. The droplet diameter has a power law dependence on the flow rate of the fuel (D∝{dot over (Q)}0.5 for JP-8 diesel (Deng et al. 2006a)) implying that the flow rate has to be decreased to reduce the droplet size. In portable power generation applications, this requirement correlates to a flow rate that is too small to be useful.
Small scale portable power systems based on the combustion of liquid hydrocarbons have become of great interest in the last decade (Epstein et al. 1997, Fréchette et al. 2003, Walther and Pisano 2003, Kyritsis et al. 2002). These combustion systems take advantage of the significantly higher energy density available in liquid hydrocarbons when compared to conventional batteries (at only 10% efficiency, diesel fuel can yield 5 MJ/kg, 10 times more than the 0.5 MJ/kg for primary batteries). Compact combustion devices in the cubic centimeter (cm3) range will likely use catalytic conversion and diffusion controlled combustion requiring the fuel to be delivered as small and rapidly evaporating droplets (Deng et al. 2006a).
Multiplexed electrospray is arraying the tubes or nozzles, thereby increasing the overall flow rate without affecting the size of the ejected droplets. In order to maximize the flow rate and miniaturize the entire system, Microelectromechanical Systems (MEMS) fabrication techniques can be used to create densely packed nozzles and integrate them with the other components. FIG. 1 depicts a prior art multiplexed electrospray atomizer with a ring extractor electrode and a grounded plane. The ring extractor acts to shield the droplet formation region (between nozzle array layer and ring extractor) from the spray region (ring extractor to ground plane) where space-charge effects become dominant. The ground plane provides a removal force to prevent droplets from flying back to the ring extractor.
Further reduction in the size of the multiplexed electrospray is currently restricted by the manual assembly technique of the components (Deng et al, 2006a). Current alignment accuracy is limited to 50 μm (microns) and prevents the assembly of smaller than conventional electrospray components. Because droplet characteristics are not affected by the changes in nozzle dimensions, improved fabrication and assembly techniques can shrink the nozzle size and increase the nozzle density. Increased nozzle density would allow further increases in device flow rate capability while maintaining sub-10 μm droplet diameters.
Thus, there exists a need to develop a process to fabricate an integrated multiplex electrospray atomizer with smaller features. There also exists a need to assemble multiplex electrospray components with greater alignment accuracy (or precision) occurring so as to promote higher electrospray flow rates with desirable droplet sizes.